Baby Unable to drink more milk. Stomach?

Hello dear Doctors, My wife has delivered a boy at 32 weeks 15 days, a pre term birth, he has been put in NICU since last 13 days, now the baby weight is 1380 GMS, doctors told me that he had some intestine infection and treated him with antibiotics and even injected white blood platelets, now the infection is cured and the only problem is that baby is not drinking enough milk, if the nurse is feeding him 15ml milk he is in taking 10ml and spitting out remaining 5ml, and this is happening when sisters are feeding him when he is asleep, we checked if he is able to suck the breast and there is no problem with his sucking ability, my only concern is about the amount of milk he is taking through the vessel. will his capacity of drinking milk increase gradually? or is he having any issue with his stomach?